JP7063414B2 - Steel plate - Google Patents

Description

The present disclosure relates to a steel sheet, and specifically to a steel sheet having a high Mn concentration, which has excellent uniform elongation characteristics, high strength, and high yield strength.

In order to achieve both weight reduction and safety of automobile bodies and parts, the strength of steel plates, which are these materials, is being increased. Generally, when the strength of a steel sheet is increased, the elongation is lowered and the formability of the steel sheet is impaired. Therefore, in order to use a high-strength steel sheet as a member for an automobile, it is necessary to enhance both strength and formability, which are contradictory characteristics. Furthermore, high-strength steel plates for vehicle body skeletons are required to have energy absorption capacity at the time of collision, and it is also important that the yield strength is high.

In order to improve uniform elongation, so-called TRIP steels utilizing the transformation-induced plasticity of retained austenite (residual γ) have been proposed (for example, Patent Document 1).

Retained austenite is obtained by concentrating C in austenite to prevent austenite from transforming into other tissues even at room temperature. As a technique for stabilizing austenite, it has been proposed that a carbide precipitation inhibitoring element such as Si and Al is contained in the steel sheet to concentrate C in the austenite during the bainite transformation occurring in the steel sheet in the manufacturing stage of the steel sheet. ing. In this technique, if the C content in the steel sheet is high, the austenite can be further stabilized and the amount of retained austenite can be increased, and as a result, a steel sheet having excellent strength and elongation can be produced.

Further, as a steel sheet having a larger amount of retained austenite than the above-mentioned TRIP steel and having a ductility exceeding the above-mentioned TRIP steel, a steel to which Mn of more than 4.0% is added has been proposed (for example, Non-Patent Document 1). Since the steel contains a large amount of Mn, the weight reduction effect on the members used thereof is also remarkable.

In Patent Document 2, a steel containing more than 4.0% Mn is cold-rolled, heated for a short time of 300 seconds to 1200 seconds, and the ferrite is controlled to 30% to 80% by area%. Discloses a steel sheet with significantly improved elongation.

In Patent Document 3, a steel containing more than 4.0% Mn, held in a temperature range of 740 ° C. or higher for 10 seconds or longer, tempered with an area% of martensite of 25% or higher and 90% or lower, and retained austenite. Disclosed is a steel sheet that ensures excellent uniform elongation characteristics and high strength by containing 10% or more and 75% or less.

Japanese Unexamined Patent Publication No. 5-59429 Japanese Unexamined Patent Publication No. 2012-237054 International Publication No. 2018/131722

Takashi Furukawa, Osamu Matsumura, Heat Treatment, Japan, Japan Heat Treatment Association, 1997, Vol. 37, No. 4, p. 204

When a steel sheet is used as a structural member, welding is often performed on the steel sheet, but if the C content in the steel sheet is high, welding becomes difficult. Therefore, it is desired to improve both the elongation and the strength of the steel sheet with a smaller C content.

Since the steel sheet described in Patent Document 2 has a structure containing a large amount of ferrite, it cannot sufficiently combine tensile strength and formability from the viewpoint of further increasing the strength and weight of the steel sheet for automobiles.

Further, although the steel sheet described in Patent Document 3 is excellent in work hardening property, there is room for further improvement in terms of yield strength in order to obtain higher impact absorption characteristics.

Therefore, a steel sheet having excellent uniform elongation characteristics, high strength, and high yield strength and having a high Mn concentration is desired.

In order to ensure excellent uniform elongation characteristics, high strength, and high yield strength in a steel plate having a high Mn content, the present inventors have added a tempered martensite phase of 25% or more in the steel plate in an area%. 90% or less and retained austenite phase should be contained in an amount of 10% or more and 75% or less, and VC (vanadium carbide) having a circle-equivalent diameter of 10 nm or more and 20 nm or less should be contained in a volume ratio of 0.30% or more and 2.20% or less. Was found to be effective.

The steel sheet of the present disclosure was made based on the above findings, and the gist thereof is as follows.

(1) By mass%,
C: More than 0.18% and less than 0.32%,
Si: 0.01% or more and less than 3.50%,
Mn: More than 4.20% and less than 6.50%,
sol. Al: 0.001% or more and less than 1.50%,
V: More than 0.10% and less than 1.20%,
P: 0.100% or less,
S: 0.010% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0% or more and less than 0.50%,
Mo: 0% or more and 2.00% or less,
W: 0% or more and 2.00% or less,
Cu: 0% or more and 2.00% or less,
Ni: 0% or more and 2.00% or less,
Ti: 0% or more and 0.300% or less,
Nb: 0% or more and 0.300% or less,
B: 0% or more and 0.010% or less,
Ca: 0% or more and 0.010% or less,
Mg: 0% or more and 0.010% or less,
Zr: 0% or more and 0.010% or less,
REM: 0% or more and 0.010% or less,
Sb: 0% or more and 0.050% or less,
Sn: 0% or more and 0.050% or less, and Bi: 0% or more and 0.050% or less, and the balance is iron and impurities.
The metallographic structure at 1/4 of the thickness from the surface in the L cross section contains a tempered martensite phase of 25% or more and 90% or less and a residual austenite phase of 10% or more and 75% or less in area%, and has a circle-equivalent diameter. A steel sheet containing VC of 10 nm or more and 20 nm or less in terms of volume ratio of 0.30% or more and 2.20% or less.
(2) By mass%,
Cr: 0.01% or more and less than 0.50%,
Mo: 0.01% or more and 2.00% or less,
W: 0.01% or more and 2.00% or less,
Cu: 0.01% or more and 2.00% or less,
Ni: 0.01% or more and 2.00% or less,
Ti: 0.005% or more and 0.300% or less,
Nb: 0.005% or more and 0.300% or less,
B: 0.0001% or more and 0.010% or less,
Ca: 0.0001% or more and 0.010% or less,
Mg: 0.0001% or more and 0.010% or less,
Zr: 0.0001% or more and 0.010% or less,
REM: 0.0001% or more and 0.010% or less,
Sb: 0.0005% or more and 0.050% or less,
Described in (1) above, further containing one or more selected from the group consisting of Sn: 0.0005% or more and 0.050% or less, and Bi: 0.0005% or more and 0.050% or less. Steel plate.
(3) The steel sheet according to (1) or (2) above, which has a hot-dip galvanized layer on the surface of the steel sheet.
(4) The steel sheet according to (1) or (2) above, which has an alloyed hot-dip galvanized layer on the surface of the steel sheet.

According to the present disclosure, it is possible to provide a steel sheet having excellent uniform elongation characteristics, high strength, and high yield strength and having a high Mn concentration.

Hereinafter, an example of the embodiment of the steel sheet of the present disclosure will be described.

1. 1. Chemical composition The reason why the chemical composition of the steel sheet of the present disclosure is defined as described above will be described. In the following description, "%" representing the content of each element means mass% unless otherwise specified.

(C: More than 0.18% and less than 0.32%)
C is an extremely important element for increasing the strength of steel and ensuring the retained austenite phase. In addition, C is also an element necessary for producing VC in this embodiment. A C content of greater than 0.18% is required to obtain a sufficient residual austenite content. On the other hand, if C is excessively contained, the weldability of the steel sheet is impaired, so the upper limit of the C content is set to less than 0.32%.

The lower limit of the C content is preferably 0.20% or more, more preferably 0.22% or more. When the C content is in the above range, the VC amount and the retained austenite amount can be more satisfactorily secured. The upper limit of the C content is preferably 0.31% or less, more preferably 0.28% or less, and by setting the upper limit of the C content in the above preferable range, the toughness of the steel sheet can be further enhanced. can.

(Si: 0.01% or more and less than 3.50%)
Si is an element effective for strengthening the tempered martensite phase, homogenizing the structure, and improving workability. In addition, Si also has an action of suppressing the precipitation of the cementite phase and promoting the residual of the austenite phase. In order to obtain the above effect, a Si content of 0.01% or more is required. On the other hand, if an excessive amount of Si is contained, the plating property and chemical conversion treatment property of the steel sheet are impaired, so the upper limit of the Si content is set to less than 3.50%.

The lower limit of the Si content is preferably 0.05% or more, more preferably 0.30% or more, still more preferably 0.50% or more. By setting the lower limit of the Si content in the above range, the uniform elongation characteristics of the steel sheet can be further improved. The upper limit of the Si content is preferably 3.00% or less, more preferably 2.50% or less.

(Mn: more than 4.20% and less than 6.50%)
Mn is an element that stabilizes the austenite phase, enhances hardenability, and ensures uniform elongation. Further, in the steel sheet according to the present embodiment, Mn is distributed in the austenite phase to further stabilize the austenite phase. More than 4.20% Mn is required to stabilize the austenite phase at room temperature. On the other hand, if the steel sheet contains an excessive amount of Mn, the manufacturability in smelting deteriorates, so the upper limit of the Mn content is set to less than 6.50%.

The lower limit of the Mn content is preferably 4.40% or more, more preferably 4.80% or more. The upper limit of the Mn content is preferably 6.00% or less, more preferably 5.50% or less. By setting the lower limit value and the upper limit value of the Mn content in the above range, the austenite phase can be further stabilized.

(Sol.Al: 0.001% or more and less than 1.50%)
Al is a deoxidizing agent and needs to be contained in an amount of 0.001% or more. In addition, Al also has an effect of improving material stability in order to widen the two-phase temperature range during annealing. The larger the Al content, the greater the effect, but if Al is excessively contained, it becomes difficult to maintain the surface texture, paintability, and weldability. The upper limit of Al is set to less than 1.50%.

sol. The lower limit of the Al content is preferably 0.005% or more, more preferably 0.01% or more, and further preferably 0.02% or more. sol. The upper limit of the Al content is preferably 1.25% or less, more preferably 1.00% or less. sol. By setting the lower limit value and the upper limit value of the Al content in the above range, the balance between the deoxidizing effect and the material stability improving effect and the surface texture, the paintability, and the weldability becomes better.

(V: More than 0.10% and less than 1.20%)
V is an element that increases the yield strength of the steel sheet by forming fine carbides and enhances the collision characteristics, and a V content of more than 0.10% is required. Further, the formation of the fine carbides improves the hydrogen embrittlement resistance. On the other hand, if V is excessively contained, the carbon required to secure the retained austenite phase is insufficient, so the upper limit of the V content is set to 1.20% or less.

The lower limit of the V content is preferably more than 0.30%, more preferably 0.32% or more, still more preferably 0.35% or more, still more preferably 0.60% or more. In particular, by setting the lower limit of the V content within the above preferable range, a larger VC content can be obtained, a steel sheet having a very excellent yield strength can be obtained, and hydrogen embrittlement resistance can be obtained. Can be improved.

The upper limit of the V content is preferably 1.10% or less, more preferably 1.00% or less. By setting the upper limit of the V content to the above range, fine carbides can be deposited and the retained austenite phase can be better secured, and the balance between uniform elongation characteristics, high strength and high yield strength of the steel sheet can be achieved. Is better, and high hydrogen embrittlement characteristics can be ensured.

(P: 0.100% or less)
P is an impurity, and if the steel sheet contains P in excess, toughness and weldability are impaired. Therefore, the upper limit of the P content is set to 0.100% or less. The upper limit of the P content is preferably 0.050% or less, more preferably 0.030% or less, still more preferably 0.020% or less. Since the steel sheet according to this embodiment does not require P, the lower limit of the P content is 0%. The lower limit of the P content may be more than 0% or 0.001% or more, but the smaller the P content is, the more preferable.

(S: 0.010% or less)
S is an impurity, and if the steel sheet contains S in excess, MnS stretched by hot rolling is generated, which causes deterioration of formability. Therefore, the upper limit of the S content is set to 0.010% or less. The upper limit of the S content is preferably 0.007% or less, more preferably 0.003% or less. Since the steel sheet according to this embodiment does not require S, the lower limit of the S content is 0%. The lower limit of the S content may be more than 0% or 0.001% or more, but the smaller the S content is, the more preferable.

(N: less than 0.050%)
N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness is lowered. Therefore, the upper limit of the N content is set to less than 0.050%. The upper limit of the N content is preferably 0.010% or less, more preferably 0.006% or less. Since the steel sheet according to this embodiment does not require N, the lower limit of the N content is 0%. The lower limit of the N content may be more than 0% or 0.003% or more, but the smaller the N content is, the more preferable.

(O: less than 0.020%)
O is an impurity, and if the steel sheet contains 0.020% or more of O, the uniform elongation property deteriorates. Therefore, the upper limit of the O content is set to less than 0.020%. The upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, still more preferably 0.003% or less. Since the steel sheet according to this embodiment does not require O, the lower limit of the O content is 0%. The lower limit of the O content may be more than 0% or 0.001% or more, but the smaller the O content is, the more preferable.

The steel sheet of the present embodiment is further selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, B, Ca, Mg, Zr, REM, Sb, Sn and Bi. The above may be contained. However, since the steel plate according to this embodiment does not require Cr, Mo, W, Cu, Ni, Ti, Nb, B, Ca, Mg, Zr, REM, Sb, Sn and Bi, Cr, Mo, W, It does not have to contain Cu, Ni, Ti, Nb, B, Ca, Mg, Zr, REM, Sb, Sn and Bi, that is, the lower limit of the content may be 0%.

(Cr: 0% or more and less than 0.50%)
(Mo: 0% or more and 2.00% or less)
(W: 0% or more and 2.00% or less)
(Cu: 0% or more and 2.00% or less)
(Ni: 0% or more and 2.00% or less)
Since Cr, Mo, W, Cu, and Ni are not essential elements for the steel sheet according to the present embodiment, they do not have to be contained, and the content of each is 0% or more. However, Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet and may be contained. In order to obtain the effect of improving the strength of the steel sheet, the steel sheet may contain 0.01% or more of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu and Ni. .. If the steel sheet contains these elements in excess, surface scratches are likely to occur during hot rolling, and the strength of the hot rolled steel sheet becomes too high, which may reduce the cold rollability. Therefore, the upper limit of the Cr content is set to less than 0.50% among the contents of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni, and Mo. , W, Cu, and Ni are set to 2.00% or less in the upper limit of each content.

(Ti: 0% or more and 0.300% or less)
(Nb: 0% or more and 0.300% or less)
Since Ti and Nb are not essential elements for the steel sheet according to the present embodiment, they may not be contained, and the respective contents are 0% or more. However, since Ti and Nb are elements that generate fine carbides, nitrides or carbonitrides, they are effective in improving the strength of the steel sheet. Therefore, the steel sheet may contain one or two elements selected from the group consisting of Ti and Nb. In order to obtain the effect of improving the strength of the steel sheet, it is preferable that the lower limit of the content of each of one or two elements selected from the group consisting of Ti and Nb is 0.005% or more. On the other hand, if these elements are excessively contained, the strength of the hot-rolled steel sheet may be excessively increased, and the cold rollability may be deteriorated. Therefore, the upper limit of the content of each of one or two elements selected from the group consisting of Ti and Nb is set to 0.300% or less.

(B: 0% or more and 0.010% or less)
(Ca: 0% or more and 0.010% or less)
(Mg: 0% or more and 0.010% or less)
(Zr: 0% or more and 0.010% or less)
(REM: 0% or more and 0.010% or less)
B, Ca, Mg, Zr, and REM (rare earth metal) are not essential elements in the steel sheet of the present disclosure and may not be contained, and the content of each is 0% or more. However, B, Ca, Mg, Zr, and REM improve the moldability by refining the MnS of the inclusions. In order to obtain this effect, the lower limit of each of one or more elements selected from the group consisting of B, Ca, Mg, Zr, and REM is preferably 0.0001% or more, more preferably 0. .001% or more. However, since an excessive amount of these elements reduces the workability of the steel sheet, the upper limit of the content of each of these elements is set to 0.010% or less, and the element is selected from the group consisting of B, Ca, Mg, Zr, and REM. The total content of one or more elements is preferably 0.030% or less.

(Sb: 0% or more and 0.050% or less)
(Sn: 0% or more and 0.050% or less)
(Bi: 0% or more and 0.050% or less)
Since Sb, Sn, and Bi are not essential elements in the steel sheet of the present disclosure, they may not be contained, and the content of each is 0% or more. However, Sb, Sn, and Bi suppress that easily oxidizable elements such as Mn, Si, and / or Al in the steel sheet are diffused on the surface of the steel sheet to form an oxide, and improve the surface texture and plating property of the steel sheet. Increase. In order to obtain this effect, the lower limit of the content of each of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005% or more, more preferably 0.001. % Or more. On the other hand, if the content of each of these elements exceeds 0.050%, the effect is saturated, so the upper limit of the content of each of these elements is set to 0.050% or less.

The rest are iron and impurities. Examples of impurities include elements that are inevitably mixed from the steel raw material, scrap, and / or the steelmaking process, and are permitted as long as they do not impair the characteristics of the steel sheet according to the present embodiment. Further, the impurity is an element other than the components described above, and includes an element contained in the steel sheet at a level at which the action and effect peculiar to the element do not affect the characteristics of the steel sheet according to the embodiment of the present invention. It is something to do.

2. 2. Metallic structure Next, the metal structure of the steel sheet according to the present embodiment will be described.

The metallographic structure in the L cross section at the 1/4 position (also referred to as 1 / 4t portion) of the thickness from the surface of the steel sheet according to the present embodiment is a tempered martensite phase having an area% of 25% or more and 90% or less, and 10 It contains a retained austenite phase of% or more and 75% or less, and contains a VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less in a volume ratio of 0.30% or more and 2.20% or less. The L cross section means a surface cut so as to pass through the central axis of the steel sheet in parallel with the plate thickness direction and the rolling direction.

(Area% of tempered martensite phase in the metal structure of 1 / 4t part of steel sheet: 25-90 area%)
The metallographic structure at a position 1/4 of the thickness from the surface in the L cross section of the steel sheet according to the present embodiment contains a tempered martensite phase of 25% or more and 90% or less in area%. The tempered martensite phase is a structure that enhances the strength of the steel sheet and improves the uniform elongation characteristics.

Within the range of the desired strength level, the area ratio of the tempered martensite phase is set to 25 to 90 area% in order to preferably maintain both the strength of the steel sheet and the uniform elongation property. If the area ratio of the tempered martensite phase is less than 25% or more than 90%, it becomes difficult to obtain sufficient strength and uniform elongation characteristics.

The lower limit of the area ratio of the tempered martensite phase is preferably 35 area% or more, more preferably 50 area% or more. If the area ratio of the tempered martensite phase is within the above preferable range, better uniform elongation characteristics are maintained even at higher strength.

The upper limit of the area ratio of the tempered martensite phase is preferably 70 area% from the viewpoint of hydrogen embrittlement.

(Area% of residual austenite phase in the metal structure of 1 / 4t part of the steel sheet: 10% or more and 75% or less)
The metallographic structure at a position 1/4 of the thickness from the surface in the L cross section of the steel sheet according to the present embodiment contains a retained austenite phase of 10% or more and 75% or less in area%. The residual austenite phase is a structure that enhances the ductility and formability of the steel sheet, particularly the uniform elongation property of the steel sheet, by the transformation-induced plasticity. Since the residual austenite phase can be transformed into a martensite phase by overhanging, drawing, stretching flange processing, or bending with tensile deformation, it contributes not only to various workability of the steel sheet but also to improvement of the strength of the steel sheet. .. In order to obtain these effects, the steel sheet according to the present embodiment needs to contain a retained austenite phase having an area ratio of 10% or more in the metal structure.

The lower limit of the area ratio of the retained austenite phase is preferably 15% or more, more preferably 18% or more, still more preferably 20% or more. When the area ratio of the retained austenite phase is within the above preferable range, better uniform elongation characteristics are maintained even at higher strength.

The larger the area ratio of the retained austenite phase, the more preferable. However, in the steel sheet having the above-mentioned chemical composition, since the solid solution carbon decreases due to VC precipitation, the area ratio of 75% is the upper limit of the area ratio of the retained austenite phase.

(The metal structure of 1 / 4t of the steel sheet contains VC with a circular equivalent diameter of 10 nm or more and 20 nm or less in volume fraction of 0.30% or more and 2.20% or less).
In the steel sheet according to the present embodiment, VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less is contained in the metal structure in a volume fraction of 0.30% or more and 2.20% or less. When a large amount of fine VC is deposited, it becomes a resistance to the movement of movable dislocations, and precipitation strengthening can be exhibited to increase the yield strength. Many of these VCs are thought to be deposited in tempered martensite. This is because tempered martensite contains a large amount of dislocations that become precipitate-forming sites as compared with ferrite, so that a larger amount of precipitate can be deposited. In order to precipitate a large amount of fine VC, it is effective to precipitate VC in the second annealing step described later. On the other hand, if the VC is deposited in the heating of the steel material (slab) before the hot rolling before the second annealing step, the winding of the hot-rolled steel sheet, and the first annealing step, the VC becomes coarse in the subsequent steps. It may be difficult to obtain the desired fine VC. Therefore, it is important not to precipitate VC in the steps prior to the second annealing step.

When the volume fraction of VC with respect to the matrix is the same, the larger the number, the finer the magnitude of VC and the higher the yield strength. In order to obtain these effects, the steel sheet of the present embodiment contains VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less in an amount of 0.30% or more and 2.20% or less in volume ratio with respect to the parent phase.

When the volume fraction of the VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less is less than 0.30%, the yield strength is insufficient. Further, in the component range of the steel sheet of the present embodiment, the upper limit of the volume fraction of the VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less is 2.20%.

The volume fraction of the VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less is preferably 0.50% or more, and more preferably 0.80% or more. When the volume fraction of VC is in the above preferable range, both uniform elongation and yield strength can be achieved at the same time.

Further, since the steel sheet according to the present embodiment contains a large amount of fine VC in the metal structure, it is excellent in hydrogen embrittlement resistance. Generally, the more diffusible hydrogen inside the steel, the worse the hydrogen embrittlement resistance. Diffusible hydrogen is trapped by vacancies, dislocations, grain boundaries or precipitates in the steel. Therefore, a steel sheet containing a large amount of dislocations and precipitates can sufficiently trap diffusible hydrogen inside the steel sheet, and thus hydrogen embrittlement cracking can be suppressed. If the volume fraction of VCs with a circle-equivalent diameter of 10 nm or more and 20 nm or less is 0.30% or more in the metal structure, a sufficient number of fine VC precipitates are present in the metal structure, so that the matching interface or misfit occurs. Dislocations increase and the amount of hydrogen traps increases, resulting in improved hydrogen embrittlement resistance. On the other hand, when the VC volume fraction having a circle-equivalent diameter of 10 nm or more and 20 nm or less is less than 0.30%, the hydrogen trap amount may be insufficient and sufficient hydrogen embrittlement resistance may not be obtained.

The circle-equivalent diameter of VC is obtained by observing a transmission electron microscope (TEM) of an extracted replica sample of a circular region with a diameter of 3.0 mm at a position 1/4 from the surface of the steel plate, and using image software to obtain a TEM image. It is measured by digitizing. As the TEM image, a randomly selected area with an area of 10 μm ² is selected. Next, the area of each particle image identified by binarization is obtained, and the circle-converted diameter of each particle is calculated based on the area. Then, among the identified particles, the particles having a diameter equivalent to a circle in the range of 10 to 20 nm are extracted. Here, when the steel sheet of the present disclosure was confirmed by energy dispersive X-ray analysis (EDS), all the particles having a circle-equivalent diameter of 10 to 20 nm were VC. Next, the total area of the particles extracted as described above, that is, the extracted VC having a circle-equivalent diameter of 10 to 20 nm is obtained, and the area ratio of the VC is calculated by dividing it by the area of the binarized image (10 μm ² ). demand. The value of the area ratio is regarded as the volume ratio of VC with respect to the matrix phase, and the volume fraction (%) of VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less is calculated. The extraction replica method is a commonly used method for exfoliating precipitates and inclusions from a metal.

The residual structure other than the tempered martensite phase and the retained austenite phase in the metal structure of the steel sheet according to the present embodiment includes a ferrite phase, a bainite phase, and a fresh martensite phase (that is, an untempered martensite phase). , Cementite phase, and tempered bainite phase.

In the steel sheet according to the present embodiment, a ferrite phase may be contained in the metal structure. From the viewpoint of ensuring uniform elongation characteristics, the area ratio of the ferrite phase in the metal structure is preferably 10% or less, more preferably 3% or less, and further preferably 0%. Therefore, for example, in the steel sheet according to the present embodiment, the area ratio of the ferrite phase in the metal structure may be 0% or more and 10% or less, or 0% or more and 3%.

Further, in the steel sheet according to the present embodiment, the bainite phase may be contained in the metal structure. The bainite phase may contain island-like martensite, which is a hard tissue. From the viewpoint of ensuring the uniform elongation property of the steel sheet, the area ratio of the bainite phase in the metal structure is preferably 5% or less, and more preferably 0%. Therefore, for example, in the steel sheet according to the present embodiment, the area ratio of the bainite phase in the metal structure may be 0% or more and 5% or less.

The method of measuring the area ratio of each phase will be described below.

(Measuring method of area% of retained austenite phase)
The area% of the retained austenite phase is measured by X-ray diffraction. A test piece having a width of 25 mm (length in the rolling direction), a length of 25 mm (length in the direction perpendicular to rolling), and a thickness in the plate thickness direction as the thickness of the annealed sample is cut out from the center of the main surface of the steel plate. This test piece is chemically polished to reduce the plate thickness by 1/4 minute to obtain a test piece having a chemically polished surface. On the surface of the test piece, X-ray diffraction analysis with a measurement range of 2θ of 45 to 105 degrees was performed three times using a Co tube, the profile of the obtained retained austenite phase was analyzed, and each was averaged. As a result, the area% of the retained austenite phase having a plate thickness of 1/4 part can be obtained. In the present embodiment, the area% of the retained austenite phase at 1/4 part of the plate thickness obtained by this method and the area% of the retained austenite phase at the L cross section are regarded as the same, and the area% obtained by this method is regarded as the same. Is the area ratio of the L cross section.

(Measurement method of area% of tempered martensite phase)
The area% of the tempered martensite phase is calculated from microstructure observation with a scanning electron microscope (SEM). After mirror-polishing the L cross section of the steel sheet, it is corroded with 3% nital (3% nitric acid-ethanol solution), and with a scanning electron microscope with an acceleration voltage of 15.0 kV and a magnification of 3000 times, 1/4 of the thickness from the surface of the steel sheet. By observing the microstructure in the range of 25 μm (length in the plate thickness direction) × 40 μm (length in the rolling direction) of the position, the area% of the tempered martensite phase can be measured.

For the tempered martensite phase, the area% is calculated by determining the white structure recognized by the observation with the scanning electron microscope whose substructure is confirmed in the crystal grains as the tempered martensite phase. ..

The measurement of the area ratio of the ferrite phase, the bainite phase, the cementite phase and the tempered bainite phase can be performed by observation with a scanning electron microscope in the same manner as the above-mentioned measurement of the area ratio of the tempered martensite phase. The ferrite phase is discriminated as a gray base structure and the area% is calculated. The bainite phase is a collection of lath-shaped crystal grains when observed with a scanning electron microscope, and is determined as a structure in which carbides extend in the same direction in the lath, and the area% is calculated. The bainite phase may also include a tempered bainite phase, but is not distinguished herein. For the cementite phase, the area of the secondary electron image taken with a brighter contrast than the other areas is defined as cementite, and the area% is calculated by image analysis.

Next, the mechanical properties of the steel sheet according to this embodiment will be described.

The TS of the steel sheet according to this embodiment is preferably 1180 MPa or more, more preferably 1470 MPa. This is because when a steel sheet is used as a material for automobiles, the thickness is reduced by increasing the strength, which contributes to weight reduction.

Further, in order to use the steel sheet according to the present embodiment for press molding, it is desirable that the uniform elongation (uEL) is also excellent. The TS × uEL of the steel sheet according to the present embodiment is preferably 21000 MPa ·% or more, more preferably 24000 MPa ·% or more, still more preferably 25,000 MPa ·% or more, still more preferably 26000 MPa ·% or more.

The yield strength of the steel sheet according to the present embodiment is preferably 800 MPa or more, more preferably 1000 MPa or more.

As described above, the steel sheet according to the present embodiment has high strength, good uniform elongation characteristics, and high yield strength, and is therefore most suitable for use in structural parts of automobiles such as pillars and cross members. Further, since the steel sheet according to the present embodiment has a high Mn concentration, it also contributes to the weight reduction of the automobile, so that the industrial contribution is extremely remarkable.

3. 3. Manufacturing Method Next, an example of a steel sheet manufacturing method according to the present embodiment will be described.

As the steel sheet according to the present embodiment, a steel having the above-mentioned chemical composition is melted by a conventional method and cast to produce a steel material (slab), which is heated and hot-rolled to obtain a hot-rolled steel sheet. Can be manufactured by pickling and then annealing.

As long as the steel sheet according to the present embodiment has the above-mentioned chemical composition, the molten steel may be melted by a normal blast furnace method, and the raw material is a large amount of scrap like steel produced by an electric furnace method. It may be included in. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.

Hot rolling can be performed on a normal continuous hot rolling line. Hot rolling is preferably carried out in a reducing atmosphere, for example, in a reducing atmosphere of 98% nitrogen and 2% hydrogen. Annealing may be carried out in either an annealing furnace or a continuous annealing line as long as the conditions described below are satisfied, but preferably, both the first annealing step and the second annealing step described later are carried out using the continuous annealing line. In that case, productivity can be improved. The first annealing step and the second annealing step are preferably performed in a reducing atmosphere, and may be performed in a reducing atmosphere of, for example, 98% nitrogen and 2% hydrogen. By heat-treating in a reducing atmosphere, it is possible to prevent scale from adhering to the surface of the steel sheet, and it can be sent to the plating process as it is without the need for pickling. Further, skin pass rolling may be performed on the steel sheet after cold rolling.

In order to obtain the metallographic structure of the steel sheet of the present disclosure, it is preferable to carry out the heat treatment conditions, particularly the annealing conditions, within the ranges shown below.

The steel material to be subjected to the hot rolling step is preferably heated before the hot rolling. The temperature of the steel material to be subjected to hot rolling (heating temperature before hot rolling) is preferably 1100 ° C. or higher and 1300 ° C. or lower. By setting the temperature of the steel material to be subjected to hot rolling to 1100 ° C. or higher, V can be solid-solved in a shorter time, and the deformation resistance during hot rolling can be further reduced. On the other hand, by setting the temperature of the steel material to be subjected to hot rolling to 1300 ° C. or lower, it is possible to suppress a decrease in yield due to an increase in scale loss. In the present specification, the temperature refers to the surface temperature of the central portion of the main surface of a steel material (slab), a hot-rolled steel sheet, or a cold-rolled steel sheet.

The time for heating to the above-mentioned preferable temperature range of 1100 ° C. or higher and 1300 ° C. or lower before hot rolling is preferably 30 minutes or longer, and more preferably 60 minutes or longer. By heating for the preferred time before hot rolling, the VC can be better dissolved and the VC can be finely precipitated in the final structure. The upper limit of the heating and holding time in the above-mentioned preferable temperature range of 1100 ° C. or higher and 1300 ° C. or lower before hot rolling is preferably 10 hours or less in order to suppress excessive scale loss, and is preferably 5 hours or less. Is more preferable. When direct rolling or direct rolling is performed, the steel material may be subjected to hot rolling as it is without being heat-treated.

In hot rolling, it is preferable to perform finish rolling. By setting the finish rolling start temperature to 1100 ° C. or lower, deterioration of the surface texture of the steel sheet due to intergranular oxidation can be suppressed.

The finish rolling end temperature is preferably 900 ° C. or higher and 1050 ° C. or lower. By setting the finish rolling end temperature within the above-mentioned preferable range, it is possible to suppress the precipitation of VC immediately after the finish rolling. The hot-rolled steel sheet obtained by finish rolling can be cooled, wound, and coiled.

The winding temperature is preferably 350 ° C. or lower. By setting the winding temperature to 350 ° C. or lower, V can be in a solid solution state, and VC precipitation in the winding step can be suppressed. The winding temperature is more preferably 200 ° C. or lower, still more preferably 100 ° C. or lower. The lower limit of the winding temperature is not particularly limited, but from the viewpoint of productivity, about room temperature may be the lower limit. After finishing rolling, cooling from 800 ° C. to 500 ° C. is preferably performed at an average cooling rate of 40 ° C./sec or higher. By setting the lower limit of the average cooling rate from 800 ° C. to 500 ° C. within the above-mentioned preferable range, the precipitation of VC can be further suppressed. The upper limit of the average cooling rate is not particularly limited, but is preferably 1000 ° C./sec or less, more preferably 200 ° C./sec or less, still more preferably 100, in consideration of suppressing the occurrence of cooling unevenness and the equipment capacity. It is ℃ / sec or less.

In order to suppress breakage during cold rolling, the hot-rolled sheet may be tempered at 300 ° C. or higher and 350 ° C. or lower after being cooled to room temperature and before cold rolling. When the hot-rolled plate tempering temperature is within the above temperature range, it is possible to obtain the effect of suppressing fracture during cold rolling without precipitating VC before cold rolling.

The hot-rolled steel sheet can be cold-rolled after being pickled by a conventional method to obtain a cold-rolled steel sheet. The rolling reduction in cold rolling is preferably 20% or more. From the viewpoint of suppressing fracture during cold rolling, the rolling reduction ratio of cold rolling is preferably 70% or less.

It is preferable to perform light rolling of about 0% to 5% before or after cold rolling and before or after pickling to correct the shape because it is advantageous in terms of ensuring flatness. Further, by performing light rolling before pickling, the pickling property is improved, the removal of the surface-concentrating element is promoted, and there is an effect of improving the chemical conversion treatment property and the plating processability.

It is preferable to heat the cold-rolled steel sheet obtained through the hot rolling step and the cold rolling step to carry out the annealing step described below. The annealing step includes a first annealing step performed after cold rolling and a second annealing step performed after final cooling in the first annealing step.

(Annealing conditions in the first annealing step: The temperature is raised from 350 ° C. to 820 ° C. or higher and the temperature at Ac 3 points or higher at an average heating rate of 10 ° C./sec or higher, and 30 seconds or longer in the temperature range of 820 ° C. or higher and Ac 3 points or higher. Retention)
In the first annealing step, the temperature is preferably raised from 350 ° C. to 820 ° C. or higher and the first annealing temperature of Ac3 or higher at an average heating rate of 10 ° C./sec or higher, and 30 in the temperature range of 820 ° C. or higher and Ac3 point or higher. Hold for more than a second. Here, the Ac3 points are C, Si, Mn, Al and V, and if any element is contained in the steel plate, the components (however, Bi, Sc, Sb, Sn) are used by using the thermodynamic calculation software Thermo Calc. , Nb and Zr are excluded), and the values obtained using TCFE8 as a reference database were used.

By setting the annealing temperature in the first annealing step to 820 ° C. or higher and Ac 3 points or higher, the parent phase can be transformed into an austenite phase to improve uniform elongation characteristics and strength, and VC that can be deposited during hot rolling can be obtained. It can be dissolved. Further, the upper limit of the annealing temperature in the first annealing step is not particularly limited, but by setting the annealing temperature to 1000 ° C. or lower, damage to the annealing furnace can be suppressed and productivity can be improved.

The annealing temperature in the first annealing step is more preferably 850 ° C. or higher, still more preferably 900 ° C. or higher, in order to further promote VC solution formation. The annealing temperature in the first annealing step is more preferably 980 ° C. or lower, still more preferably 950 ° C. or lower.

In the first annealing step, the average heating is preferably 10 ° C./sec or higher, more preferably 15 ° C./sec or higher, preferably in the temperature range of the first annealing temperature (820 ° C. or higher and Ac 3 points or higher) from 350 ° C. It is preferable to raise the temperature at a rate. By setting the lower limit of the average heating rate within the above-mentioned preferable range, precipitation or coarsening of VC during temperature rise can be suppressed, and solution formation in the first annealing step can be promoted. The upper limit of the average heating rate is not particularly limited, but from the viewpoint of suppressing uneven heating of the steel sheet and the equipment capacity, it is 30 ° C./sec or less in the temperature range of 350 ° C. to 820 ° C. or higher and Ac 3 points or higher. It is preferable to do so.

In the first annealing step, it is preferable that the annealing time at the first annealing temperature is 30 seconds or more in order to sufficiently austenite the matrix and to dissolve the precipitate. The annealing time is more preferably 40 seconds or more. The upper limit of the annealing time is not particularly limited, but from the viewpoint of productivity, the annealing time is preferably 300 seconds or less.

(Cooling conditions after annealing in the first annealing step: Cooling to a temperature range of 350 ° C or lower)
In the cooling after annealing in the first annealing step, it is preferably cooled from the first annealing temperature to 350 ° C. or lower. By setting the final cooling temperature after annealing in the first annealing step to 350 ° C. or lower, precipitation of VC during cooling can be suppressed.

More preferably, the final cooling temperature after annealing in the first annealing step is less than 100 ° C. This makes it possible to increase the rasmartensite structure immediately after the first annealing step. From the viewpoint of ensuring safety during transportation of the steel sheet, the final cooling temperature after annealing in the first annealing step is preferably room temperature (50 ° C. or lower).

In the cooling in the first annealing step, since the steel plate is quenched to promote the martensitic transformation, the temperature range from the annealing temperature in the first annealing step to 350 ° C. is preferably cooled at an average cooling rate of 10 ° C./sec or more. do. By setting the average cooling rate in the temperature range from the first annealing temperature to 350 ° C. (hereinafter, also referred to as the average cooling rate after annealing) to 10 ° C./sec or higher, the precipitation of VC during cooling can be suppressed.

The average cooling rate after annealing in the first annealing step is preferably 20 ° C./sec or more, more preferably 50 ° C./sec or more, still more preferably 200 ° C./sec or more, still more preferably 250 ° C./sec or more. .. By setting the average cooling rate after annealing within the above-mentioned preferable range, the entire steel material after cooling can be cooled at a critical cooling rate or higher, and the entire cooled steel material can have a martensite-based structure, so that V can be kept in a solid solution state. It is possible to control the structure after the final heat treatment and improve the material stability.

The upper limit of the average cooling rate after annealing in the first annealing step is not particularly limited, but even if a water quenching cooling method or a mist injection cooling method is used, it is difficult to control the temperature to over 2000 ° C./sec, so after annealing. The practical upper limit of the average cooling rate is 2000 ° C./sec.

In the cooling after annealing in the first annealing step, the cooling shutdown temperature of the average cooling rate in the above range is preferably 350 ° C. or lower, more preferably 200 ° C. or lower, still more preferably 100 ° C. or lower. By cooling at an average cooling rate in the above range and setting the cooling shutdown temperature within the above temperature range, VC precipitation after cooling can be suppressed.

(Holding conditions after cooling stop in the first annealing step: Hold for 10 seconds or more and 1000 seconds or less in the temperature range of 350 ° C or lower)
Preferably, after cooling after annealing in the first annealing step, it is held in a temperature range of 350 ° C. or lower for 10 seconds or more and 1000 seconds or less. By setting the temperature holding time after cooling stop in the above temperature range to 10 seconds or more, C distribution to austenite proceeds sufficiently, and austenite is more generated in the structure before the final heat treatment (before the second annealing step). be able to. As a result, it is possible to further suppress the formation of austenite in the structure after the final heat treatment and further suppress the fluctuation of the strength characteristics. On the other hand, even if the holding time is more than 1000 seconds, the effect of the above action is saturated and the productivity is lowered. The holding time in the above temperature range is more preferably 30 seconds or more. From the viewpoint of productivity, the holding time in the above temperature range is more preferably 300 seconds or less.

In the first annealing step, the lower limit of the holding temperature after cooling stop in the above temperature range is not particularly limited, but the holding temperature after cooling stop is preferably 50 ° C. or higher, more preferably 100 ° C. or higher, still more preferably 200 ° C. or higher. This makes it possible to improve the efficiency of the continuous annealing line. On the other hand, VC precipitation can be suppressed by setting the holding temperature to 350 ° C. or lower after the cooling is stopped. During the holding time, the temperature of the steel sheet does not need to be constant as long as the holding temperature range is 350 ° C. or lower. Further, it is not always necessary to hold the product in the holding temperature range after cooling.

(Annealing conditions in the second annealing step: held for 50 seconds or more and 360 seconds or less in a temperature range of 640 ° C or higher and 720 ° C or lower)
After cooling after annealing in the first annealing step, after holding in a temperature range of preferably 100 ° C. or higher and 350 ° C. or lower, the steel sheet is cooled to less than 100 ° C., preferably to room temperature, and then heated again to perform the second annealing step. I do. In the second annealing step, it is preferably held for 50 seconds or more and 360 seconds or less in a temperature range of 640 ° C. or higher and 720 ° C. or lower.

By setting the second annealing temperature to 640 ° C. or higher, VC can be sufficiently precipitated and the yield strength can be increased. Further, by setting the second annealing temperature to 720 ° C. or lower, a sufficient amount of tempered martensite can be secured, a sufficient amount of VC precipitation can be secured, and a sufficient yield strength and uniform elongation can be secured. ..

The second annealing time is set to 50 seconds or more in order to stabilize the retained austenite and secure the amount of VC precipitation. The second annealing time is preferably 100 seconds or longer, more preferably 200 seconds or longer. Further, in order to suppress the coarsening of VC, the second annealing time is set to 360 seconds or less.

In the second annealing step, preferably, when heating in the temperature range of 640 ° C. or higher and 720 ° C. or lower, the temperature range from 500 ° C. to 600 ° C. is set to an average heating rate of 10 ° C./sec or higher and 200 ° C./sec or lower. The temperature rises. By setting the average heating rate from 500 ° C to 600 ° C in the second annealing to 10 ° C / sec or more, the formation of cementite in the tissue is suppressed, and C required for stabilization of retained austenite and precipitation of VC is obtained. It can be secured more reliably. Further, by raising the temperature in the temperature range of 500 ° C. to 600 ° C. at an average heating rate of 200 ° C./sec or less, temperature unevenness of the steel sheet is less likely to occur, and more stable quality can be ensured.

(Cooling conditions after annealing in the second annealing step: Cooling to a temperature range of 350 ° C or lower at an average cooling rate of 10 ° C / sec or higher)
Preferably, after holding in the temperature range of 640 ° C. or higher and 720 ° C. or lower in the second annealing step, the steel sheet is cooled to 350 ° C. or lower at an average cooling rate of 10 ° C./sec or higher. By keeping the average cooling rate from the second annealing temperature to 350 ° C. within the above-mentioned preferable range, coarsening of VC can be suppressed. The average cooling rate is the average cooling rate in the temperature range from the holding temperature in the second annealing step to 350 ° C. When the hot-dip galvanizing treatment and / or the alloying treatment described later is performed by stopping the cooling in the middle, the average cooling rate is calculated without considering the time required for these treatments.

If the steel sheet is not plated, the cooling after annealing in the second annealing step may be performed as it is up to room temperature. Further, when plating a steel plate, the following can be performed.

When hot-dip galvanized steel sheet is manufactured by hot-dip galvanizing the surface of the steel sheet, cooling after annealing in the second annealing step is stopped in the temperature range of 430 to 500 ° C., and then the cold-rolled steel sheet is made of hot-dip galvanized steel sheet. The hot-dip galvanizing treatment can be performed by immersing in a plating bath. The conditions of the plating bath may be within the normal range. After the plating treatment, it may be cooled to room temperature, preferably to 100 ° C. or lower at an average cooling rate of 30 ° C./sec or more. Alternatively, after cooling after annealing in the second annealing step to a temperature range of 350 ° C. or lower, the temperature of the cold-rolled steel sheet is raised to a temperature range of 430 to 500 ° C., and the cold-rolled steel sheet is used as a hot-dip galvanizing bath. It can also be immersed and hot-dip galvanized. When hot-dip galvanizing is performed, the difference between the annealing temperature in the second annealing step and the final temperature reached by cooling after plating is set to the cooling time from the second annealing step to the start of plating and the final temperature after the end of plating. By dividing by the sum of the cooling time until reaching the point, the average cooling rate after annealing in the second annealing step can be obtained.

When the surface of a steel sheet is hot-dip galvanized to produce an alloyed hot-dip galvanized steel sheet, the temperature is 450 to 620 ° C. after the steel sheet is hot-dip galvanized and before the steel sheet is cooled to room temperature. Hot dip galvanizing can be alloyed at temperature. The alloying treatment conditions may be within the normal range. After the alloying treatment, it may be cooled to room temperature, preferably to 100 ° C. or lower at an average cooling rate of 30 ° C./sec or higher. When the alloying process is performed after hot-dip zinc plating, the difference between the annealing temperature in the second annealing step and the final temperature reached by cooling after the alloying process is the cooling time from the second annealing step to the start of plating and alloying. The average cooling rate after annealing in the second annealing step can be obtained by dividing by the sum of the cooling time from the end to the final temperature.

The above-mentioned manufacturing method is an example of the manufacturing method of the steel sheet of the present disclosure, and the manufacturing method of the steel sheet of the present disclosure is not limited to the above-mentioned manufacturing method.

The steel sheet of the present disclosure will be described more specifically with reference to an example. However, the following examples are examples of the steel sheet of the present disclosure and its manufacturing method, and the steel sheet of the present disclosure and its manufacturing method are not limited to the embodiments of the following examples.

1. 1. Production of Steel Sheet for Evaluation The steel having the chemical composition shown in Table 1 was melted in a converter and continuously cast to obtain a slab having a thickness of 245 mm.

The obtained steel material (slab) was subjected to heat treatment, hot rolling, winding, and tempering under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. Then, the hot-rolled steel sheet after winding or tempering was cold-rolled. The hot rolling and heat treatment of the hot-rolled steel sheet were performed in a reducing atmosphere of 98% nitrogen and 2% hydrogen. In all the examples, the holding time at the heating temperature before hot rolling was 60 minutes, and the cold rolling ratio was 40%.

The obtained cold-rolled steel sheet was annealed twice (first annealing step and second annealing step) under the conditions shown in Table 3 to prepare a baked cold-rolled steel sheet. The two annealings of the cold-rolled steel sheet were carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen.

For some examples of hot-dip cold-rolled steel sheets, cooling after the second annealing is stopped at 460 ° C, and the cold-dip steel sheet is immersed in a hot-dip zinc plating bath at 460 ° C for 2 seconds to perform hot-dip galvanizing treatment. rice field. The conditions of the plating bath were the same as those of the conventional one. When the alloying treatment described later was not performed, the mixture was cooled to room temperature at an average cooling rate of 30 ° C./sec after holding at 460 ° C. Regarding the "average cooling rate from the second annealing temperature to 350 ° C. or lower" in the example shown as "plating" in Table 3, the difference between the second annealing temperature and the room temperature in Table 3 is the difference after the second annealing step. It was calculated by dividing by the sum of the cooling time from the start of plating to the start of plating and the cooling time from after plating to reaching room temperature.

For some examples of annealed cold-rolled steel sheets, after hot-dip galvanizing treatment, they were subsequently alloyed without being cooled to room temperature. It was heated to 520 ° C. and held at 520 ° C. for 5 seconds for alloying treatment, and then cooled to room temperature at an average cooling rate of 30 ° C./sec. Regarding the "average cooling rate from the second annealing temperature to 350 ° C. or lower" in the example shown as "alloying" in Table 3, the difference between the second annealing temperature and the room temperature in Table 3 is the difference in the second annealing step. It was obtained by dividing by the sum of the cooling time from the later to the start of plating and the cooling time from the alloying treatment to reaching room temperature.

The annealed cold-rolled steel sheet thus obtained was tempered and rolled at an elongation rate of 0.1% to prepare various evaluation steel sheets.

2. 2. Evaluation method The annealed cold-rolled steel sheets obtained in each example were subjected to microstructure observation, tensile test, and uniform elongation test, and tempered martensite area ratio, ferrite area ratio, retained austenite area ratio, and bainite area ratio. , VC volume ratio with a circle-equivalent diameter of 10 to 20 nm, tensile strength (TS), uniform elongation property (TS × μEL), and yield strength (YS) were evaluated. The method of each evaluation is as follows.

(Area ratio of each phase)
The area ratios of the tempered martensite phase, ferrite phase, retained austenite phase, and bainite phase were calculated from microstructure observation and X-ray diffraction measurement with a scanning electron microscope. The L cross section of the steel plate cut parallel to the plate thickness direction and the rolling direction is mirror-polished, and then the microstructure is exposed with 3% bainite. The microstructure at the position was observed, and the area ratios of the tempered martensite phase, ferrite phase, and bainite phase were calculated by image analysis (Photoshop®) for a range of 0.1 mm × 0.3 mm. Further, a test piece having a width of 25 mm (length in the rolling direction), a length of 25 mm (length in the direction perpendicular to rolling), and a thickness in the plate thickness direction as the thickness of the annealed sample is obtained from the center of the main surface of the steel sheet. The test piece was cut out and chemically polished to reduce the plate thickness by 1/4 minute to obtain a test piece having a chemically polished surface. On the surface of the test piece, X-ray diffraction analysis with a measurement range of 2θ of 45 to 105 degrees was performed three times using a Co tube, the profile of the obtained retained austenite phase was analyzed, and each was averaged. As a result, the area% of the retained austenite phase having a plate thickness of 1/4 part was obtained. The area% of the retained austenite phase at 1/4 part of the plate thickness obtained by this method is regarded as the same as the area% of the retained austenite phase at the L cross section, and the area% obtained by this method is the area of the L cross section. It was a rate.

(VC yen-converted diameter and volume fraction)
The circle-equivalent diameter of VC is obtained by observing a transmission electron microscope (TEM) of an extracted replica sample of a circular region with a diameter of 3.0 mm at a position 1/4 from the surface of the steel plate, and using image software to obtain a TEM image. It was measured by digitizing. As the TEM image, a randomly selected area with an area of 10 μm ² was selected. Next, the area of each particle image identified by binarization was obtained, and the circle-equivalent diameter of each particle was calculated based on the area. Then, among the identified particles, the particles having a circle-equivalent diameter in the range of 10 to 20 nm were extracted. Here, when each steel sheet was confirmed by energy dispersive X-ray analysis (EDS), all the particles having a circle-equivalent diameter of 10 to 20 nm were VC. Next, the total area of the particles extracted as described above, that is, the extracted VC having a circle-equivalent diameter of 10 to 20 nm is obtained, and the area ratio of the VC is calculated by dividing it by the area of the binarized image (10 μm ² ). I asked. The value of the area fraction was regarded as the volume fraction of VC with respect to the matrix phase, and the volume fraction (%) of VC having a circle-equivalent diameter of 10 nm or more and 20 nm or less was calculated.

(Tensile test / uniform elongation test method)
A JIS No. 5 tensile test piece was taken from a direction perpendicular to the rolling direction of the steel sheet, and the tensile strength (TS), uniform elongation (uEL), and yield strength (YS) were measured. The tensile test was performed by the method specified in JIS-Z2201 using a JIS No. 5 tensile test piece. The uniform elongation test was carried out by the method specified in JIS-Z2201 using a JIS No. 5 test piece having a parallel portion length of 50 mm.

3. 3. Evaluation Results Table 4 shows the results of the above evaluation. A steel sheet exhibiting a tensile strength (TS) of 1180 MPa or more, a TS × uEL of 21000 MPa ·% or more, and a yield strength (YS) of 800 MPa or more is evaluated as a steel sheet having excellent uniform elongation characteristics, high strength, and high yield strength. bottom.

The hydrogen embrittlement resistance of the above example numbers 10, 11, 31, 33 and 47 was evaluated. The evaluation method is as follows.

(Evaluation method of hydrogen embrittlement resistance)
From the steel plates of Example Nos. 10, 11, 31, 33 and 47, three test pieces punched to 30 mmφ with a clearance of 10% were collected, and the punched test pieces were immersed in a pH 1 hydrochloric acid aqueous solution for 48 hours to obtain a punched end face. The presence or absence of cracks was observed with an optical microscope. All three test pieces were accepted if no cracks were observed after 48 hours of immersion.

The results of the above evaluation are shown in Table 5. A steel sheet that did not show cracks in all three test pieces after immersion for 48 hours was evaluated as a steel sheet having excellent hydrogen embrittlement resistance, and was shown as "○" in Table 5 with hydrogen embrittlement resistance. A steel sheet showing cracks even by one is shown in Table 5 as having a hydrogen embrittlement resistance property of "x".

Claims (4)

By mass%,
C: More than 0.18% and less than 0.32%,
Si: 0.01% or more and less than 3.50%,
Mn: More than 4.20% and less than 6.50%,
sol. Al: 0.001% or more and less than 1.50%,
V: More than 0.10% and less than 1.20%,
P: 0.100% or less,
S: 0.010% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0% or more and less than 0.50%,
Mo: 0% or more and 2.00% or less,
W: 0% or more and 2.00% or less,
Cu: 0% or more and 2.00% or less,
Ni: 0% or more and 2.00% or less,
Ti: 0% or more and 0.300% or less,
Nb: 0% or more and 0.300% or less,
B: 0% or more and 0.010% or less,
Ca: 0% or more and 0.010% or less,
Mg: 0% or more and 0.010% or less,
Zr: 0% or more and 0.010% or less,
REM: 0% or more and 0.010% or less,
Sb: 0% or more and 0.050% or less,
Sn: 0% or more and 0.050% or less, and Bi: 0% or more and 0.050% or less, and the balance is iron and impurities.
The metallographic structure at 1/4 of the thickness from the surface in the L cross section contains a tempered martensite phase of 25% or more and 90% or less and a residual austenite phase of 10% or more and 75% or less in area%, and has a circle-equivalent diameter. A steel sheet containing VC of 10 nm or more and 20 nm or less in terms of volume ratio of 0.30% or more and 2.20% or less. By mass%,
Cr: 0.01% or more and less than 0.50%,
Mo: 0.01% or more and 2.00% or less,
W: 0.01% or more and 2.00% or less,
Cu: 0.01% or more and 2.00% or less,
Ni: 0.01% or more and 2.00% or less,
Ti: 0.005% or more and 0.300% or less,
Nb: 0.005% or more and 0.300% or less,
B: 0.0001% or more and 0.010% or less,
Ca: 0.0001% or more and 0.010% or less,
Mg: 0.0001% or more and 0.010% or less,
Zr: 0.0001% or more and 0.010% or less,
REM: 0.0001% or more and 0.010% or less,
Sb: 0.0005% or more and 0.050% or less,
The first aspect of claim 1, further comprising one or more selected from the group consisting of Sn: 0.0005% or more and 0.050% or less, and Bi: 0.0005% or more and 0.050% or less. Steel plate. The steel sheet according to claim 1 or 2, which has a hot-dip galvanized layer on the surface of the steel sheet. The steel sheet according to claim 1 or 2, which has an alloyed hot-dip galvanized layer on the surface of the steel sheet.